Note from the Fed Reserve of Philly:

In January of each year, the Federal Reserve Bank of Philadelphia revises historical data for the Business Outlook Survey after calculating new seasonal adjustment factors. Because survey responses are characterized by seasonal patterns, new seasonal factors are calculated annually based on the addition of a complete year of data.

Money quote:

The largest revision for the current activity index was for the month of August — the index was revised up 8 points (from the extraordinarily low one-month reading of -30.7, to a revised reading of -22.7).

Link to the historical changes charts here.